On 12/31/2015 01:19 PM, William G. Scott wrote:
> Dear Ed:
>
> If you instead were to report chemical composition in terms of activity, in addition to conceptual rigor, it will possess the added advantage of being unit-free, so no one can pin you down regarding which non-standard set of units to use, and you should also suggest to the reviewers and/or editors that a simple application of fifth-order Extended Debye-Hückel theory should be sufficiently adequate to facilitate conversion to more low-brow units.
>
OK, that's easy. I define unit activity as activity in the solid phase. Which solid phase? whichever crystal form or precipitate happens to develop. So after everything comes to equilibrium the activity of my protein in solution is 1. What if first one crystal form grows, then later a more stable form nucleates and sucks the protein out of the first? Activity is 1, then undefined, then 1 again. This convention makes the solubility of any material 1, which is very convenient.
> Peace and joy for the new year.
>
> Bill
>
>
>
> William G. Scott
>
> http://scottlab.ucsc.edu
>
>> On Dec 31, 2015, at 8:34 AM, Edward A. Berry <[log in to unmask]> wrote:
>>
>> Reviewer#2:
>> In addition, the authors' incessant use of "g/L" for concentration is irritating. For one thing, it gives a deceptive false impression of the scale these reactions are being carried out on, since most of the volumes are a few milliliters or less. Furthermore, it is conventional to express concentration of detergents (Triton X-100, C12E10) or cell culture media (typtone, yeast extract, dextrose) in units of percent, so the reader is forced to convert g/l to percent in order to compare with previous experience.
>>
>> Author #1:
>> Concentration is an intrinsic property, independent on the total amount of material, and the units should not change with the volume.
>> Are you satisfied with molarity defined as mols/L? or should we have M1 for mols/L, M2 for mmol/ml, and M3 for umol/ul?
>> . .
>> It's kind of like speed in miles per hour (or km/h), and most people don't have any problem with that. Would you want extra calibration rings on your car's speedometer dial giving the speed in yards/min (for driving in parking lots) and feet/sec (for driveways)?
>>
>> If your neighbor says he was backing out of his driveway at 5 mph when (something happened), do you get the deceptive false impression that his driveway must be at least 5 miles long? Or that they must have been backing out for at least an hour? Of course not. You have a good feel for what kind of speed 5 mph is, and you probably have no trouble envisioning something moving at 5 mph for 2 sec and then coming to a stop.
>>
>> That reminds me of the one about the guy who tried to get out of paying a speeding ticket. He took it to court and told the judge, "Your Honor, the nice officer is obviously mistaken if he says he clocked me doing 50 MPH on Elm street. Elm street is only seven miles long, and I had only been driving for 5 minutes! It would appear that he had his radar gun calibrated for interstate traffic, and any result obtained on a short street like Elm is invalid". (And then there is the one where Prof. Doppler tried to get out of a ticket for running a red light- but that is a different story - he ended up getting a speeding ticket instead.)
>>
>> If you try to envision how fast 60 MPH is, do you first envision a distance of 60 miles, and then think how fast something would have to be going to cover that distance in an hour? Probably not. You just have an idea how fast 60 MPH is, from experience.
>> I have a good idea how concentrated protein is at 5 g/l, and I can envision pipetting 10 ul of such a solution into a cuvet or an eppendorf tube. When I do that, does its concentration suddenly change from 5 g/l to 5 ug/ul? (My algebra teacher used to take off points if I turned in a solution like "1000 y = 5000 x".) If it is 100 g/l it might be a bit viscous, and I would give extra time to make sure the correct volume ran in. But I don't figure this out by saying, "100 ug/ul is like 100 mg/ml
>> is like 100 g/l". I just call that concentration 100 g/l .
>>
>> In fact we _do_ think about total weight and total volume when we make solutions, but that is still easy with g/l if you multiply the units along with the numbers. Make 100 ul of 5g/l protein? 100 ul x 5 g/l = 500 ug you need to weigh out. (use a good balance!) More often we will be diluting from a stock solution: I want 5 g/l detergent and the stock solution is 200 g/l (incorrectly labeled 20% w/v), so that is a 40-fold dilution. For 100 ul, I use 2.5 ul. the math would be the same if I wanted 5 mg/ml and the stock solution had been 200 mg/ml, or 0.5% w/v from a 20% w/v stock,. There is no advantage to using submultiples or deprecated "% w/v" in the calculation.
>>
>>
>> Author #2:
>> Author #1 is correct in saying the units for weight concentration should not depend on the volume, however since this journal recommends the use of SI units, which are based on the meter-kg-sec (MKS) system of units, the proper unit for mass concentration is kg/m^3 (look up "mass concentration" in wikipedia). We have therefore everywhere replaced g/L with kg/m^3 (and mg/L, ug/L by g/m^3, mg/m^3). Although "kigs-per-em-cubed" does not roll off the tongue quite as easily as gram-per-liter or even "Migs-per-em-el", that is a small price to pay for SI correctness. We hope this resolves the issue to everyone's satisfaction.
>> =======================
>> All fictional of course, but i have had an editor go through my manuscript and everywhere replace g/L with mg/ml without consulting.
>> Happy New Year,
>> eab
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